The book presents a detail study on searching of potential metal cluster motifs, e.g., aluminum, beryllium clusters and their various complex compounds, useful for designing novel nanostructures. The efficacy of such cluster units to be used as possible lead components of molecular electronic devices is also investigated in detail employing a newly proposed reactivity descriptor, viz., nucleo(electro)philic excess. The behavior of DFT based local and global reactivity descriptors around the transition state of various thermochemical reactions are studied in detail. The potential of quantum chemical reactivity descriptors in explaining biological activity of various sex hormones, e.g., testosterone, estrogen etc. and toxicity of a series of aliphatic and aromatic compounds including Polyaromatic hydrocarbons (PAHs) are explored in detail.